Many conventional hot melt adhesives are thermoplastic materials. Hot melt adhesives typically exist as solids at room temperature and can be converted into a flowable liquid upon application of heat and/or pressure. To form a bond between adherends, the adhesive is first converted into a softened or molten state. The adhesive is then allowed to solidify upon cooling to form the bond. In comparison to thermosetting adhesives, i.e., adhesives that undergo chemical crosslinking upon curing, hot melt adhesives possess many advantages. One major advantage is that hot melt adhesives may be formulated without any liquid carrier. Also, bonding occurs immediately upon cooling, although improved bond properties for semi-crystalline hot melt adhesives may develop over minutes or days as crystallization progresses. Bonds formed from thermoplastic materials are also repairable in that they can usually be removed and replaced without damage to the substrate(s) being bonded.
The adhesive matrix properties of a hot melt adhesive are key factors in determining the performance and reliability of a hot melt adhesive bond between substrates that are to be bonded together. The adhesive should flow sufficiently at a suitable bonding temperature to allow the substrate surfaces to be adequately wetted by the adhesive. In addition, the adhesive composition should bond rapidly upon cooling. When the bonding operation is complete, the adhesive should have sufficient toughness to provide good resistance to peel, and should have high enough modulus and creep resistance sufficient to withstand the expected stress and/or high or low temperatures, at least through the range of use. In some cases, it is further desirable that the adhesive be reworkable at a processing temperature that is sufficiently low so that the substrate is not damaged during removal and replacement. The adhesive should have an extended shelf life at room temperature (i.e., is storage-stable). The adhesive may need sufficiently low moisture absorption to maintain adequate performance (electrical resistance, peel strength, etc.) at conditions of elevated temperature and humidity.
Hot melt adhesive compositions are used in a wide range of home and industrial applications. Examples of such applications include package sealing, wood bonding, plastic assembly such as home crafts, fabric bonding, and bonding flexible circuits to substrates.
One illustrative application of hot melt adhesives involves their formulation and use as electrically conductive adhesives, especially anisotropically electrically conductive adhesives. Anisotropically electrically conductive adhesive compositions often include electrically conductive particles dispersed in a polymeric adhesive. These adhesive compositions have the ability to establish multiple, discrete, electrical connections, often in extremely close proximity, between two substrates. The compositions can be used to mechanically bond and electrically interconnect an electrical component on the first substrate to an electrical component on the second substrate. The electrical contact is primarily a pressure-engaged contact and is maintained by the adhesive matrix holding the two substrates together. A typical use is to electrically couple the contacts in a flex circuit to the corresponding multiple contacts of a printed circuit board.
Commonly, these adhesives may be delivered as a layer on a release liner or as free standing films in which the polymeric adhesive matrix contains a sufficient amount of the conductive particles to allow electrical conduction through the thickness (the z-axis) of the film after bonding. Because of the insulating nature of the adhesive polymer matrix, and the low volume fraction of conductive particles, the films allow essentially no conductivity in the plane of the film (x and y-axis directions). Such film types also are known as “z-axis adhesive films” or “ZAF's.”
Anisotropically electrically conductive adhesive films additionally containing non-electrically conductive fillers, are known in the literature. For example, Japanese Patent Kokai Publication No. 3-223380 discloses an anisotropic conductive film that includes 20% by volume aluminum oxide or aluminum nitride with a particle size of 10 to 20 micrometer in addition to containing electrically conductive particles.
Japanese Patent Kokai Publication No. 2-206670 discloses an anisotropic conductive film containing 3 to 50% by volume of a thermally conductive filler having an average particle size of 1 micrometer to 50 micrometer in addition to containing electrically conductive particles.
Japanese Patent Kokai Publication No. 62-177082 discloses an anisotropic conductive film containing 1 to 30% by weight silica particles having an average primary particle size of 4 micrometer to 100 micrometer. The conductive particles are metal-clad organic polymer cores.
U.S. Pat. No. 5,672,400 describes the use of a semi-crystalline thermoplastic as the adhesive matrix for a ZAF. Key features of this adhesive are a relatively high modulus over the use temperature range (associated with low tendency to creep) and a relatively low modulus (or viscosity) at the bonding temperature (associated with fast bonding since the bond time is limited by the time for the adhesive to flow out of the way and allow the conductive substrates to make connections to the electrically conductive particles). Fillers are listed that can be used to provide increased adhesion, higher modulus, and decreased coefficient of thermal expansion. These fillers include silica particles, silicate particles, quartz particles, ceramic particles, glass bubbles, inert fibers, and mica particles. Organophilic clays are not suggested.
It is highly desirable for the adhesive of an anisotropically electrically conductive adhesive composition to provide fast bonding times as well as rapid development of creep resistance and peel strength. In the past, both thermosetting and thermoplastic adhesive polymers have been used in electrically conductive adhesive compositions. Thermosets, or more specifically thermoset adhesives, tend to flow readily during the initial phases of bonding (i.e. prior to curing) due to their relatively low molecular weight, and have high creep resistance and peel strength once cured, due to the chemical crosslinks in the system, but suffer from relatively long bonding times before such characteristics develop adequately. As another drawback, thermoset adhesives may not be repairable, i.e., they cannot be removed from a substrate without leaving a residue or degrading the substrate surface.
In contrast to thermosetting materials, thermoplastics, more specifically thermoplastic adhesives, can bond more rapidly than thermoset adhesives since no time is needed to allow the adhesive to cure. However, most thermoplastics do not have sufficient creep resistance and peel strength to perform adequately in some applications in which anisotropically electrically conductive adhesive compositions are used. In addition, many thermoplastics do not exhibit sufficient flow at elevated temperatures to enable very rapid bonding.
Semi-crystalline thermoplastics are a desirable subset of thermoplastics for use in hot-melt adhesives, including ZAF's. The crystalline regions of the semi-crystalline thermoplastic serve as physical crosslinks, which allow the material to perform more like a thermoset below the melting point of the crystalline regions. Above the melting point of the crystalline regions, the material flows readily, enabling rapid bonding.
However, one problem affecting conventional anisotropically electrically conductive adhesive compositions concerns resistance against the formation of cavitation voids during bonding. When a ZAF or other anisotropically electrically conductive adhesive composition is used to bond two electronic substrates together, it is common for pressure to be applied along with heat during bonding. In one typical bonding method, a heated block (hot bar) or thermode is used to apply both heat and pressure. As used herein, the term “thermode” is also intended to include hot bars, heated blocks, etc. This bonding pressure can deform one or both of the substrates. For example, when a flex circuit is bonded to a printed circuit board (PCB), the flexible circuit substrate (e.g. polyester, polyimide, etc.) may deform, particularly in the region between circuit lines. When the pressure is released, the deformed substrate can resiliently spring back toward its original configuration. Cavitation voids within the body of the adhesive polymer matrix can form as a result, because the internal strength of the adhesive typically is not high enough to resist the elastic recovery of the deformed substrate(s) when the thermode is lifted hot. Cavitation voids are desirably avoided, as these can make it more difficult to evaluate bond quality and are often perceived as being a potential source of failure and/or loss in performance.
Cavitation voids are not so easily avoided with conventional adhesive compositions. With respect to thermosets, crosslinking the adhesive in order to avoid formation of cavitation voids cannot be done prior to making the bond, as excessive crosslinking will prevent the adhesive from flowing and making the desired electrical contact. Crosslinking the adhesive during the bonding process (by incorporation of reactive sites that can react during bonding) tends to result in bond times that are too long to be practical or cost effective in high speed commercial fabrication processes. On the other hand, thermoplastics generally do not have sufficient internal strength immediately after bonding to resist the formation of cavitation voids as the thermode is lifted. Cooling the bondline while still maintaining pressure can be used to reduce the formation of cavitation voids. However, this undesirably increases the length of time required to make a bond. Clearly, some strategy for alleviating cavitation voids is needed.
Thus, further improvements in the performance of adhesive polymers based on semi-crystalline thermoplastics are desired in order to make them perform more like thermosets without requiring substantially longer bond times. It would be highly desirable to have an adhesive polymer that has the benefits of both thermosetting and thermoplastic materials, namely a combination of fast bonding times and rapid development of sufficiently high creep resistance and peel strength to be suitable for use in a wider range of applications, e.g., use as anisotropically electrically conductive adhesive compositions.